In offshore well drilling operations, the operator must pass tools through remote well components that have surfaces specially prepared for eventual sealing with another well component. For example, the well component might be a wellhead housing having seal surfaces for a packoff of a casing hanger and possibly also a tubing hanger. During drilling, the drill bits and drill strings must pass through the wellhead housing, thus could damage the seal surfaces.
Normally, the operator installs a wear bushing over the seal surfaces in the wellhead housing. A wear bushing is a sleeve, normally metal, that is placed over the seal surfaces. Usually, the wear bushing is run on a running tool lowered on pipe, such as drill pipe. When the operator is ready to install the component in the seal surface or to change to a smaller drill bit size, he must retrieve the wear bushing. In deep water, the time to run and retrieve a wear bushing is very costly.
Wear bushings that are run in with the drill bit and retrieved with the drill bit are known and will reduce the cost of a trip but are not used extensively because of possible malfunctions. Also, once either type of wear bushing has been removed, there is no protection for the seal surface until the component has landed and sealed against the seal surface. For example, the lower end of the casing below a casing hanger could come into contact with the seal surface while the casing is being run. U.S. Pat. No. 6,691,921 shows a powered centralizer located above a wellhead housing for centering equipment being lowered through the wellhead housing. The system shown therein is not yet in use, and improvements are desirable.